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Method for a pad design with a variety of shapes

Publishing Venue

Abstract

Disclosed is a method for a pad design with a variety of shapes. Benefits include improved performance and improved reliability.

Country

United States

Language

English (United States)

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Method for a pad design with a variety of shapes

Disclosed is a method for a
pad design with a variety of shapes. Benefits include improved
performance and improved reliability.

Background

The size of chip components is scaling down and input/output (I/O)
density is increasing. This chip scaling leads an increased incidence of
tombstoning and bridging. Tombstoning occurs when the melt times of two pads
differ. It also occurs when the wetting forces of two pads differ.

Before reflow, solder paste tackiness holds the component
during the chip mounting and board transferring (see Figure 1). The figure
illustrates the following values:

• F1, F2 =
Tackiness from solder paste

• F3 = Weight of
chip component

• X = Distance
from component center of gravity

During the reflow, wetting force from the molten solder plays
an important role. The difference in temperature on the two pads at any time
during the reflow will lead to the imbalance moment. In the case of difference
in temperature, tombstone occurs if the following equation is true (see Figure
2): F3 (X) < F5 cos α (X) + F4 sin θ (X)

which simplifies to:

F3 < F5 cos α + F4
sin θ

The equation above contains the following values:

• F2 = Tacky force of solder paste

• F3 = Weight of chip component

• F4, F5 = Wetting force of molten solder at two different edges of the terminal

Bridging occurs when the pastes of two adjacent pads are in
close proximity. Typically, paste is splashed away from its original location
and goes near another pad when placed by a placement machine with force. The
problem is becoming increasingly critical because smaller part sizes require
closer placement.

The conventional
solution is to reduce the stencil aperture opening on a solder-masked pad to
reduce the amount of paste deposited on the pads. For example, a 1:1 stencil
aperture may be changed to an oval shape. As a result, the difference in
pulling force between two terminals is reduced.